A Wireless-Implantable Microsystem for Continuous Blood Glucose Monitoring

Abstract

Recent studies have shown that continuous blood glucose monitoring can help to signifi­cantly reduce the symptoms associated with diabetes. This project seeks to develop an im­plantable microsystem for continuous monitoring of blood glucose concentration. The ma­jor challenges towards this goal are the microsystem size, the in-vivo lifetime of the mi­crosystem and the in-vivo functionality of the glucose sensor used in the microsystem. The size of the microsystem plays a key role in increasing the period of consistent glucose monitoring in vivo and also in simple implantation of the microsystem with an outpatient surgery. In this research work, a novel process was proposed to minimize the implant size to about 4x8x 1 rnm3. Firstly, the entire interface circuitry was integrated in an integrated circuit chip; secondly, the sensor, required for glucose sensing, was microfabri­cated using micromachining techniques; and thirdly, flip-chip bonding was used to attach the transponder chip to the glucose sensor. In order to increase the in-vivo lifetime and functionality of the microsystem, a novel oxygen-free electrochemical biosensor was used in the microsystem. Also, a passive telemetry link was implemented to provide the power to the microsystem and avoid the lifetime problem associate with using battery power for the microsystem. The glucose sensor used in the microsystem is an electrochemical biosensor, which does not require oxygen for reaction with glucose. More importantly, it does not produce hydrogen peroxide, which is a major cause in the degradation of the sensor lifetime. The microsystem receives inductive power from an external reader and transmits the measured glucose data to the external reader using load shift keying. Using passive telemetry for data and power transfer results in a smaller implant, potentially more com­fort for the patient, and a mechanically more robust implant. It also promises simplified packaging of the implant and it avoids the hazards associated with using a battery or a wired link for powering the implant. The integrated circuit chip, required for interfacing with the glucose sensor, was fabricated using the TSMC 0.18 µm CMOS process, has a total area of 1.3x 1.3 mrn2 and consumes a total current of about 110 µA. It needs three off-chip components to be func­tional: a coil inductor and two capacitors. The chip properly functions providing that the amplitude of the received RF signal from the external reader is higher than 2.6 V. Research on different circuit blocks of the integrated chip resulted in several state­of-the art circuits, which can be used for similar implantable sensors and also for similar applications such as radio-frequency identification.